To understand the mechanism of contraction in a muscle it is necessary to know how the contractile proteins are organized and how they interact during contraction. In striated muscle, knowledge of the organization of contractile proteins was obtained mainly by observing cells before and after differential extraction or antibody staining. Other measurements, such as birefringence, x-ray diffraction, and periodicities of filaments became more meaningful and valid once the general organization was shown. Studies of smooth muscle organization have by-passed these two essential techniques, localization by fluorescent probes such as labelled antibodies and differential extraction, mainly due to the difficulty in obtaining suitable isolated cells. Due to development of techniques for isolating smooth muscle cells by the principal investigator and others, these studies are now feasible and should provide essential information. The purpose of the research will be to use localization techniques to determine the organization of contractile proteins in both relaxed and contracted smooth muscle cells. As many preparations as possible will be used so the generalizations will be broad, and also so that structure-function relationships can be determined. It is hoped that the information obtained will allow development of a comprehensive model of contraction in smooth muscles, and that it will ultimately allow structural diagnosis of malfunctioning contractile mechanism. Such information should provide a better basis for diagnosis and devising treatment of disorders, such as asthma and some types of hypertension, where smooth muscle contraction is a significant factor.